Double cusp gyro gun

ABSTRACT

A gyrotron gun that generates gyrating electron beams in a controllable manner suitable for use in a wide range of gyro-amplifiers and gyro-oscillators is disclosed. The gyrotron comprises first and second means for abruptly changing a magnetic field and which means are positioned between first, second and third field coils. The field coils are operated so as to provide for a desired magnetic field profile that allows for the control of the parameters desired to provide for small-orbit, large-orbit, and linear modes of operation of the gyrotron gun. The gyrotron gun further comprises of a pair of bucking coils arranging near the cathode to independently control the axial velocity spread of the gyrating electron beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the generation of electron beams foruse in microwave generators and, more particularly, to the generation ofgyrating electron beams in a controllable manner particularly suited foruse in a wide range of gyro-amplifiers and gyro-oscillators.

2. Description of the Prior Art

Gyro-amplifiers and gyro-oscillators, which are commonly referred to asgyrotrons, require an electron beam that is different from that which isnormally employed in linear microwave tubes, such as klystrons andtraveling-wave tubes. In general, and as is known in the art, ingyrotrons microwave energy is extracted from the beam rotational energythrough electron orbital phase bunching resulting from resonantinteraction between the electron gyration and the transverse componentof the electromagnetic waves contained in the interaction circuit,sometimes referred to as cyclotron resonant maser instability. Tomaximize this resonant interaction process and enhance the efficiency ofthe gyrotron, it is necessary for the bem forming system, also known asgyrotron gun or gyro-gun, to accomplish the following three factors: (a)form the gyrating electron beam with a large transverse-to-axialvelocity ratio, α=v_(⊥)/v_(z) typically between one and two; (b) achieveand control the axial velocity, v_(z), so as to have a low velocityspread in order to provide phase bunching stability; and (c) place theelectron's beam guiding center, r_(g), at the peak of the wavetransverse electric field. The attainment of these three factors for allgyrotron device applications has not been accomplished by a singlegyro-gun because of certain limitations.

First, the desired factors of the transverse-to-axial velocity ratio,α=v_(⊥)/v_(z), and the desired position of the electron's beam guidingcenter, r_(g), primarily determine the electron orbital parameter whichis different for various gyrotron applications. The selection of thesedesired factors in a gyro-gun to satisfy a gyrotron device requiring aparticular orbit, such as, a small-orbit (non-axis encircling), may notbe suitable when the same gyrotron gun is used for another applicationrequiring a different type of orbit, such as, a large-orbit(axis-encircling).

Second, the desired parameter of controlling the axial velocity v_(z)spread is primarily of importance to the interaction circuit located atthe output of the gyrotron gun and which circuit extracts microwaveenergy from the kinetic energy of the gyrating electron. The axialvelocity spread is determined, in part, by the cathode of the gyrotrongun and the operation of the cathode. One approach to control the axialvelocity v_(z) spread is to reduce the cathode's annulus width which, inturn, has the disadvantage of creating higher cathode loading. Anotherapproach is to increase the cathode's mean radius which, in turn, hasthe disadvantage of increasing the overall size of the gyrotron gunwhich may not be desired for some applications.

FIRST PROBLEM

In relation to the first problem, several approaches, dictated by theparametric requirements in the beam-wave interaction region, have beenused to provide for a desired small-orbit or large-orbit gyrotron beam.Although gyrotron guns designed for a particular parametric requirementserve well their intended function, once built employing existingbeam-forming techniques, the gyrotron gun is often difficult to adjustin order to accommodate parameter changes that may arise from time totime. Various beam-forming devices determined by parametric requirementsthat have been developed prior to 1981 are well documented andsummarized in a detailed report by Baird and Attard entitled “GyrotronGun Study Report” of the Naval Research Laboratory (NRL) Report TR-3-476(1981). Each of the approaches prior to 1981 is suitable for use as abeam-forming system for a specific gyrotron device, depending upon thetype of beam parameters required. For instance, the gyrotron gun,magnetic injection gun (MIG), originally conceived in the early 1960's,has been continuously used until the present time as a gyrotronbeam-forming system and is particularly suited for small-orbitapplications, but is not suited as a gyrotron beam-forming system havinglarge-orbit applications. For a large-orbit gyrotron applications, amodified version of a magnetically shielded, space-charged limitedPierce gun (known in the art) has been proposed and is described by G.P. Scheitrum; R. S. Symons; and R. B. True, in the technical articleentitled “Low Velocity Spread Axis Encircling Electron Beam FormingSystem,” documented in the Technical Digest of Electron Devices Meeting,pp 743-746 (1989). Accordingly, although various beam-forming techniquesare known to accommodate both small and large-orbit gyrotron devices, noone technique is known to accommodate both the small and large-orbitapplications.

SECOND PROBLEM

In relation to the second problem, a primary cause of axial velocityv_(z) spread in gyrotron devices is due to the fact that electronsemitted from the cathode of the gyrotron gun at different radialpositions enclose different amounts of magnetic flux, commonly referredto as canonical angular momentum spread. As previously mentioned,several approaches are known to reduce the axial velocity v_(z) spreadand one of which is to reduce the cathode's annulus width. This is nothowever very practical, since this reduction creates a higher cathodeloading factor, which has a tendency to overburden the cathode and,thereby, degrade its operational life characteristic. Another approachis to increase the cathode's mean radius. While this approach reducesvelocity spread, it is accomplished at the expense of increasing theoverall size of the gyrotron gun which may not be desired for someapplications. An approach is to reduce the axial magnetic field on thesurface of the cathode. An adaptation of this approach is to use amagnetic envelope and a magnetic center post as proposed by Chow andPantell in the technical article “The Cyclotron Resonance Backward WaveOscillator,” documented in the proceedings of the IEEE, Vol. 48, pp.1865-1867 (1980). In this technique, the center post carries themagnetic flux, while the magnetic envelope reduces the axial magneticfield on the cathode structure to virtually zero. However, a problemwith this technique is that the magnetic center post is at essentiallythe same potential as that of the cathode; hence, practicalimplementations of this technique are prone to arcing between the centerpost and the anode due to large potential differences at theirproximity. Moreover, this approach does not permit the flexibility ofvarying the beam canonical angular momentum spread to actively controlthe beam velocity spread for different applications of the gyrotron gun.

OBJECTS OF THE INVENTION

Accordingly, one object of the present invention, the double cuspgyro-gun, is to provide a gyrotron gun and a method of use thereof thathave the flexibility of actively controlling the axial velocity v_(z)spread so as to accommodate different applications of the gyrotron gun.

Another object of the present invention is to provide a gyrotron gun anda method of use thereof that actively control the gyrating electron'sbeam transverse-to-axial velocity, α=V_(⊥)/v_(z), as well as theposition of the electron's beam guiding center, r_(g), so as to allowthe gyrotron gun to be used for both small and large orbitingapplications.

A still further object of the present invention is to provide a gyrotrongun and a method of use thereof that provide the flexibility forindependently and simultaneously controlling the gyrating electron beamtransverse-axial velocity ratio, α=V_(⊥)v_(z); the position of theelectron's beam guiding center, r_(g); as well as the spread of theaxial velocity, v_(z).

SUMMARY OF THE INVENTION

The invention is directed to a gyrotron gun that is operated toindependently and simultaneously control a gyrating electron beamtransverse-to-axial velocity ratio, α=v_(⊥)v_(z); the position of theelectron's beam guiding center, r_(g); as well as the spread of theaxial velocity v_(z), thereby, allowing the gyrotron gun to be used forlarge-orbit, small-orbit and even linear-beam modes of operation.

The gyrotron gun generates and forms a beam of electrons manifestingelectron gyrating around a guiding center and having rotational energy.This is so that efficient phase bunching will result from a resonantinteraction between the electron gyration and transverse component ofthe electromagnetic wave in the ensuing beam-wave interaction circuit.The gyrotron gun comprises first, second and third field coils and firstand second means for establishing an abrupt change in a magnetic field.The field coils and the devices for establishing an abrupt change arearranged to form three regions. The field coils are operated so thateach supply a predetermined strength of an axial magnetic field to allowfor the control of the gyrating electron beam transverse-to-axialvelocity ratio, α=V_(⊥)/v_(z); the position of the electrons beamguiding center r_(g); and the spread of the axial velocity, v_(z), sothat the gyrotron gun can be used for small and large-orbit and evenlinear-beam modes of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention, as well as the invention itself, will become betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein likereference numerals designate identical or corresponding plots throughoutthe several views, and wherein:

FIG. 1 is a schematic illustrating an arrangement of the interrelatedelements of the present invention;

FIG. 2 illustrates a plot of the magnetic profile of the presentinvention;

FIG. 3 is composed of FIGS. 3(A), (B), and (C), wherein FIGS. 3(A) and3(B) each illustrates a plot useful in the understanding in thelarge-orbit operation of the gyrotron gun of the present invention whichis generally illustrated in FIG. 3(C);

FIG. 4 is composed of FIGS. 4(A), (B), and (C), wherein FIGS. 4(A) and4(B) each illustrates a plot useful in the understanding of thesmall-orbit operation of the gyrotron gun of the present invention whichis generally illustrated in FIG. 4(C);

FIG. 5 is composed of FIGS. 5(A), (B), and (C), wherein FIGS. 5(A) and5(B) each illustrates a plot useful in the understanding of thelinear-mode operation of the gyrotron gun of the present invention whichis generally illustrated in FIG. 5(C);

FIG. 6 is composed of FIGS. 6(A), and (B), each of which illustrates aplot related to the axial velocity, v_(z), spread associated with thelarge-orbit operation of the gyrotron gun of the present invention;

FIG. 7 is composed of FIGS. 7(A), (B), and (C), each of whichillustrates a plot related to the axial velocity, v_(z), spreadassociated with the small-orbit operation of the gyrotron gun of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 illustrates an arrangement ofelements of a gyrotron gun 10 having a centerline 12. The gyrotron gun10 is preferably circular so that the arrangement of the elements ofFIG. 1 is actually also below the centerline 12.

The gyrotron gun 10 generates gyrating electron beams in a controllablemanner suitable for a wide range of applications. The gyrotron gun 10has a predetermined operating period, sometimes referred to as agyro-period, with a corresponding predetermined wavelength. The gyrotrondevice 10 may also be referred to herein as a double-cusp gyro gun 10.The gyrotron device 10 generates and forms a beam of electronsmanifesting electron gyrating around a guiding center and havingrotational energy. This is so that efficient phase bunching will resultfrom a resonant interaction between electron gyration and a transversecomponent of electromagnetic waves contained in the ensuing beam-waveinteraction circuit 50. The resonant interaction is determined by threefactors which are: (1) the transverse-to-axial velocity ratio,α=v_(⊥)/v_(z), of the gyrating electron beam; (2) the position of theelectron's beam guiding center, r_(g), and (3) the spread of the axialvelocity v_(z) of the electron beam. The gyrotron device 10 of FIG. 1comprises a plurality of elements each having a reference number all ofwhich are given in Table 1.

TABLE 1 REFERENCE NO. ELEMENT 14 Cathode 16 Anode 18 Vacuum Envelope 20Electron Tunnel 22 Entrance Section of Electron Tunnel 20 24Intermediate Section of Electron Tunnel 20 26 Exit Section of ElectronTunnel 20 28 First Field Coil 30 Second Field Coil 32 Third Field Coil34 First Cusp Member 36 Second Cusp Member 38 First Bucking Coil 40Second Bucking Coil 42 Electron Beam

The cathode 14 is preferably of a thermionic type or any other electronemitting type and also preferably has an annular shape. As is known, thethermionic cathode 14, when subjected to a relatively high-voltage pulse44, generated from a conventional power modulator 46 and present at theentrance section 22, allows the extraction therefrom of an electron beam42 which is accelerated toward a higher potential anode 16, also locatedin the entrance section 22. The electron beam 42 moves along the fulllength of an electron tunnel 20 and is extracted from the exit section26 thereof, as shown by arrow 48, into a beam-wave interaction circuit50, known in the art. The beam-wave interaction circuit 50 converts thekinetic energy of the electron beam 42 into microwave energy.

Also as is known in the art, and as is shown in FIG. 1, the anode 16(preferably of an annular shape) is offset from the cathode 14 andprotrusions 52 and 54 are interposed therebetween allowing the electronbeam 42 attracted to the anode 16 to be diverted away therefrom anddirected toward the intermediate section 24.

The vacuum envelope 18, illustrated by a cross-hatch representation inFIG. 1, forms the electron tunnel 20 having the entrance section 22, theintermediate section 24 having first and second portions 24A and 24B,and the exit section 26. The vacuum envelope 18 is preferably formed ofa non-magnetic metallic material and arranged in a manner known in theart.

The first, second and third field coils 28, 30, and 32, respectively,are preferably formed of a metallic material, known in the art, and arearranged so that the first field coil 28 is situated near the entrancesection 22 and the second and third field coils 30 and 32 are situatedat the intermediate section 24. The field coils 28, 30 and 32, as wellas the bucking coils 38 and 40, are indicated in FIG. 1 with an Xsymbol.

The first and second cusp devices 34 and 36 preferably comprise a highpermeability material, such as soft iron. The term “cusp” is known inthe art and is meant to represent that the device provides for an abruptchange in a magnetic field as the magnetic field passes through the cuspdevice. The arrangement of the cusp devices 34 and 36, relative to thefield coils 28, 30 and 32, provide for three different operating regionswhich are of particular importance to the present invention.

The first cusp device 34 is interposed between the first and secondfield coils 28 and 30, respectively. The second cusp device 36 isinterposed between the second and third field coils 30 and 32,respectively. The first field coil 28 and the first cusp device 34establish a first operating region 56. Further, the first cusp device 34and the second cusp device 36, both in combination with the second fieldcoil 30, establish a second operating region 58. Further, the secondcusp device 36 and the third field coil 32 establish a third operatingregion 60. The first (56), second (58), and third (60) operating regionsare respectively herein termed the diode region 56, the double-cuspregion 58, and the adiabatic compression region 60. The first cuspdevice 34 and the second cusp device 36 are spaced apart, as shown inFIG. 1, from each other by a distance 62 which preferably corresponds toone-half of the operating gryo-period of the gyrotron device 10.

The first and second bucking coils 38 and 40 are supplied with oppositecurrents and are located near, but preferably behind, the cathode 14, asshown in FIG. 1, so as to reduce or even cancel the axial magnetic fieldon the cathode surface in a manner as to be further describedhereinafter with reference to FIGS. 6 and 7.

The parameters of the electron beam 42 formed and generated by thegyrotron gun 10 are controlled by the field strength of the axialmagnetic fields applied to the regions 56, 58 and 60 by way of thefirst, second and third field coils 28, 30 and 32, respectively. Theoperation of the gyrotron device 10 is primarily determined by amagnetic profile 64, which may be further described with reference toFIG. 2.

FIG. 2 illustrates the magnetic profile 64 of the electron beam 42 as itpasses through and is developed therein by the diode, double-cusp andadiabatic compression regions 56, 58 and 60, respectively, andpropagates to the tip 66 of the exit section 26, whereby the kineticenergy of the electron beam 42 is extracted by the beam-wave interactioncircuit 50 to provide microwave energy therefrom. FIG. 2 has ahorizontal axis, given by the quantity, z, representative of the axialposition of the electron beam 42 as it moves along the gyrotron device10, as generally illustrated in FIG. 1. Further, FIG. 2 has a verticalaxis represented by the quantity B_(z) (Z) which is the average axialmagnetic field enclosed by the electron beam 42.

The magnetic profile 64 comprises a first section 64A, a second section64B, and a third section 64C, respectively, associated with regions 56,58 and 60, and respectively represented by the quantities f₁B₀, f₂B₀ andB₀ and B_(f)=f_(m)B₀, wherein the quantities B₀ and B_(f)=f_(m)B₀ arepresent at the initial and terminal portions, respectively, of the thirdsection 64C. The cusp devices 34 and 36 (see FIG. 1) impart a rotationto the electron beam 42 which impartations are generally illustrated inFIG. 2 by ramping portions 64D and 64E respectively.

In general, the usage of the first and second cusp devices 34 and 36,the selection of the quantities f₁B₀, f₂B₀, B₀ and f_(m)B₀, determinedby the operation of the first, second and third field coils 28, 30 and32 respectively, as well as the operation of the bucking coils 38 and40, provide for independent and simultaneous control of the threefactors: (1) the transverse-to-axial velocity ratio, α=v_(⊥)/v_(z); (2)the position of the electron's beam guiding center, r_(g); and (3) theaxial velocity spread, all of which three factors have been previouslydiscussed in the “Background” section. The operation and method of thepresent invention provide the capability to optimize the energyconversion in the beam-wave interaction circuit 50 without modifying themechanical features of the double cusp gyro gun 10. More particularly,once the physical features, composition and arrangement of the elementsof the double-cusp gyro gun 10 are selected, they do not need to bechanged, but rather only the amount of current applied to the fieldcoils 28, 30 and 32 and bucking coils 38 and 40 need to be adjusted tocontrol the operating parameters of the double-cusp gyro gun 10. Thus,problems and limitations associated with prior art beam-formingpractices, discussed in the “Background” section, are avoided.

The trajectory of the electrons forming beam 42, after extraction fromthe cathode 14, is dictated primarily by the magnetic field profile 64,via Lorentz force. Evaluation of such a trajectory is based on anidealized theoretical model which employs the principle of canonicalangular momentum conservation. The canonical angular momentum, P_(θ), ofan electron charge q, mass m and energy γ, at radius r, axial positionz, and angular velocity v_(θ) may be represented by expression 1 givenbelow:

P _(θ) =γmrv _(θ) −qrA _(θ)(r,z)  (1)

The term P_(θ) represent a conserved quantity in an azimuthallysymmetric system. The vector potential, A_(θ)(rz), for the totalmagnetic field profile 64, may be estimated by expression 2 given below:

A _(θ)(r,z)=½rB _(z)(z)  (2)

where B_(z) (z) is the average axial magnetic field enclosed by anelectron forming part of the electron beam 42. With the quantityω_(c2)≡(qB_(z))/(γm), the canonical angular momentum may be representedby expression 3 given below which is treated as a conserved quantitythroughout the entire gyrotron device 10:

 P _(θ)=(γm)[rv _(θ)−½r ²ω_(cz)(z)]  (3)

As previously mentioned with reference to FIG. 2, the first cuspdevice's 34 transition field imparts an angular velocity to the electronbeam 42, via the v_(z)×B_(r) Lorentz force term, hence, initiatingcyclotron motion within the double-cusp region 58. From conservation ofthe canonical angular momentum, it can be shown that the electronperpendicular velocity in the double-cusp region 58 may be representedby expression 4 given below:

v _(⊥1+)=½(1−f ₁)f ₂ω_(co) r ₁  (4)

where r₁ is assumed to be the electron radial position at the cathode 14and also at the first cusp device 34; f₁ and f₂ are the axial magneticfield quantities respectively related to the diode region 56 and thedouble cusp region 58, and ω_(co) is the electron angular cyclotronfrequency at the start of the adiabatic compression region 60. Moreparticularly, with reference to FIG. 2, the quantities f₁ and f₂ arerespectively included in sections 64A and 64B of magnetic profile 64 andthe quantity ω_(co) is included in the peak of the ramp portion 64E ofthe magnetic profile 64. It should be pointed out that the electronperpendicular motion immediately after the first cusp device 34 isprimarily azimuthal, since the electron motion has assumed to beparaxial in the diode region 56. Based on this assumption, the electronposition at the second cusp device 36 where the electron radial velocityvanishes may be represented by the expression 5 given below:

r ₂ =f ₁ r ₁  (5)

As previously mentioned, the distance 62 (see FIG. 1) between the twocusp devices 34 and 36 is preferably and precisely one-half of agyro-period. For such a preferred distance 62, the electron motion inthe double-cusp region 58 is that of a small-orbit (non-axis encircling)gyration wherein its guiding center may be represented by expression 6given below:

r _(g1+)=½(1+f ₁)r ₁  (6)

At the second cusp device 36, the radial magnetic field thereat impartsan additional velocity thrust on the electron beam (see FIG. 2, inparticular, ramp portion 64E). As further seen in FIG. 2, the fieldstrength at the exit of the double-cusp region 58 is B₀ which isillustrated near the termination of the ramp portion 64E whichcorresponds to the second cusp device 36 transition region of theprofile 64. Upon leaving the transition region of the second cusp device36, the perpendicular velocity may be represented by expression 7 givenbelow:

 V _(⊥2+)=½(f ₁ −f ₂)ω_(co) r ₁  (7)

The guiding center region at the beginning of the adiabatic compressionregion 60 may be represented by expression 8 given below:$\begin{matrix}\begin{matrix}{r_{{g2} +}\quad = {r_{2} - \frac{V_{\bot{2 +}}}{\omega_{co}}}} \\{\quad {= {{1/2}\left( {f_{1} + f_{2}} \right)r_{1}}}} \\\quad\end{matrix} & (8)\end{matrix}$

It should be noted, and as will be further discussed hereinafter withreference to FIGS. 6 and 7, that for a particular case wherein f₁=−f₂,there is no beam guiding center spread. The ratio of the perpendicularvelocity to axial velocity is found from energy conservation and may berepresented by expression 9 given below: $\begin{matrix}{\alpha_{2 +} = {\frac{v_{\bot{2 +}}}{\left\lbrack {v_{o}^{2} - v_{\bot 2} +^{2}} \right\rbrack^{1/2}}\quad \quad = \left\lbrack \frac{{1/4}\left( {f_{1} - f_{2}} \right)^{2}{\omega^{2}}_{co}r_{1}^{2}}{v_{o}^{2} - {{1/4}\left( {f_{1} - f_{2}} \right)^{2}\omega_{co}^{2}r_{1}^{2}}} \right\rbrack^{1/2}}} & (9)\end{matrix}$

where v_(o) is the electron velocity at the exit of the diode region 56.

From expressions (7) and (8) it may be shown that by properly selectingthe magnetic field profiles, in particular the quantities f₁ and f₂, awide variety of beam configurations can be generated and are shown inTable 2.

TABLE 2 QUANTITIES f₁ AND f₂ OF MAGNETIC FIELD PROFILE 64 BEAMCONFIGURATION (f₁ = f₂) Linear Beam (|f₁| = −|f₂|) Large-Orbit (f₁ ≠ f₂)Small-Orbit

The present invention also provides a means to control independently thebeam guiding center r_(g), and the transverse-to-axial α=v_(⊥)/v_(z) viaselecting the sum, f₁+f₂, and the difference, f₁−f₂, of quantities ofdiode region 56 and of double-cusp region 58.

Finally, after the adiabatic compression region 60 alters the axialfield strength from B₀ to B_(f)=f_(m)B₀, the guiding center r_(g)sometimes referred to as r_(gf), and transverse-to-axial velocity ratioα, sometimes referred to as α_(f), at the tip 66 (see FIG. 2) of thegyrotation device 10 as it enters the beam-wave interaction circuit 50may be respectively represented by expressions 10 and 11 given below:$\begin{matrix}{r_{gf} = {{\frac{1}{f_{m}^{1/2}}r_{{g2} +}} = {\frac{f_{1} + f_{2}}{2f_{m}^{1/2}}r_{1}}}} & (10) \\{\alpha_{f} = {\left\lbrack \frac{\left( \alpha_{2 +} \right)^{2}f_{m}}{1 + {\left( {1 - f_{m}} \right)\left( \alpha_{2 +} \right)^{2}}} \right\rbrack^{1/2} = \left\lbrack \frac{{1/4}{f_{m}^{- 1}\left( {f_{1} - f_{2}} \right)}^{2}\omega_{cf}^{2}r_{1}^{2}}{v_{0}^{2} - {{1/4}{f_{m}^{- 1}\left( {f_{1} - f_{2}} \right)}^{2}\omega_{cf}^{2}r_{1}^{2}}} \right\rbrack^{1/2}}} & (11)\end{matrix}$

where ω_(cf) is the angular cyclotron frequency at the r_(f) interactionregion of the beam-wave interaction circuit 50.

In addition to the relationships given by the expressions 10 and 11 forterms normally referred to as r_(g) and α, for a gyrotron gun 10satisfying the requirements that the distance 62 between the two cuspdevices 34 and 36 (see FIG. 1) being one-half of a gyro-period inlength, the gyrotron device 10 further has interrelationship between thequantities f₂ and f_(m). That is, the ratio f₂/f_(m) is determined byboth f₂B₀ (half gyro-period criterion), and f_(m)B₀ (interaction circuit50 requirement). Consequently, for a given value of the parameter B_(f),it is possible to adjust the guiding center, r_(g), and thetransverse-to-axial velocity ratio, α, independently by adjusting thequantities f₁ and f₂. More particularly, the employment of thedouble-cusp region 58 in the gyrotron gun 10 permits the independentcontrol over the parameters (α, r_(g), and B) in a relatively simplemanner by means of selecting and adjusting the quantities f₁, f₂, andf_(m), hence, achieving more flexibility with less complexity ascompared to prior art beam-forming apparatuses.

It is important to emphasize, however, that the necessity that theradial velocity vanish (see expression (5)) at the second cusp device 36(hence, the half-gyro-period length criterion discussed for distance 62)is not really needed for small-orbit operations of the gyrotron gun 10.However, for linear and large-orbit operations of the gyrotron gun 10,it is desired that the radial velocity be zero (see expression (5)) atthe second cusp device 36 so as to ensure that beam ripple (scalloping)is minimized.

It should now be appreciated that the practice of the present inventionprovides for a gyrotron gun 10 employing a first and second cusp devices34 and 36, respectively, that allow for the ability to provideindependent and simultaneous control of the quantities of expressions 10and 11, that is, guiding center r_(g) and transverse-to-axial velocityratio α respectively. Furthermore, it should be appreciated that thecontrol of the guiding center r_(g) and transverse-to-axial velocityratio α is accomplished simply by varying the magnetic field profile 64shown in FIG. 2 and is done so without modifying or altering anyphysical features of the gyrotron gun 10.

In the practice of the present invention, a beam optic simulation studywas performed. In the study, the mechanical features of the gyrotron gun10 remain fixed and only the magnetic field profile 64 was varied toaffect various final beam parameters that allowed the gyrotron gun 10 toprovide small and large-orbits and linear beam modes of operation. Themagnetic field profiles were obtained from a magnetic design code,POISSON, by specifying the currents for the electric field coils 28, 30and 32, and the bucking coils 38 and 40 all shown in FIG. 1. The POISSONis a well-known magnetic design code developed by Los Alamos NationalLaboratories. The magnetic field profiles obtained from the POISSONmagnetic design were used as inputs to a MAGIC code to perform beamoptics simulation. The MAGIC code is a self-consistent, two-and-one-halfdimensional, particle-in-cell code developed by Mission ResearchCorporation and is known in the art. The study performed for thegyrotron device 10 resulted in different beam types exemplified by threebeam optic cases shown in FIGS. 3, 4 and 5 and respectivelyrepresentative of a large-orbit operation, a small-orbit operation, anda linear beam mode of operation.

FIG. 3 is composed of FIGS. 3(A), (B) and (C), wherein FIGS. 3A and 3Brespectively illustrates the beam perpendicular γβ_(p) and axial momentaγβ_(z), normalized by the speed of light as a function of axialdistance. A gyrotron gun 10 is generally illustrated in FIG. 3(C), butwithout the placement of the field coils 28, 30 and 32 thereon. FIGS.3(A), (B) and (C) are all interrelated to the diode region 56,double-cusp region 58, and the adiabatic compression region 60 (shownabove FIG. 3(A)) and the interrelationship thereof is shown by the useof dimensional lines 68 and 70. FIG. 3 shows a beam perpendicularmomentum plot 72 (FIG. 3(A), an axial momentum plot 74 (FIG. 3(B)) and abeam trajectory 76 (FIG. 3(C)), all corresponding to the usage of thegyrotron gun 10 for a large-orbit operation, where the resulting beamtrajectory 76 is rotated around the gyrotron axis 12 (axis encircling).The large-orbit operation of FIG. 3 was accomplished by the use of a 60kV, 4.4-A electron beam, which was also used in the operationsillustrated in FIGS. 4 and 5.

FIG. 4 is similar to FIG. 3 and illustrates a plot 78 of the beamperpendicular momentum quantity γβ_(p) (FIG. 4(A)), a plot 80 of theaxial momentum quantity γβ_(z) (FIG. 4(B)) and a beam projectory 82(FIG. 4(C)), all related to the small-orbit operation of the gyrotrongun 10. As is known in the art, for a small-orbit operation, theelectrons comprising electron beam 42 of FIG. 1 are rotated around andoff-axis from its guiding center r_(g).

FIG. 5 is similar to both FIGS. 3 and 4 and illustrates a plot 84 of thebeam perpendicular momentum quantity γβ_(p) (FIG. 5(A)), a plot 86 ofthe axial momentum quantity γβ_(z) (FIG. 5(B)), and a beam projectory 88(FIG. 5(C)), all related to a linear-beam mode of operation of thegyrotron gun 10. The linear-beam operation is one in which the beam isnon-rotating. It should be noted in FIG. 5(A) that the perpendicularmomentum γβ_(p) essentially vanishes after the second cusp device 36(not shown) that separates the double-cusp region 58 from adiabaticcompression region 60.

It should now be appreciated that the practice of the present inventionprovides for a gyrotron gun 10 wherein the quantities given in Table 2may be selected so as to provide for a small-orbit, large-orbit orlinear modes of operation.

As mentioned in the “Background” section, the energy conversionefficiency of the beam-wave interaction circuit 50 is dependent upon thebeam velocity spread. As further discussed in the “Background” section,various approaches were used to control the beam velocity spread, butnone yielded complete success. The present invention accomplishes suchcontrol by the use of the bucking coils 38 and 40. More particularly,the bucking coils 38 and 40 are supplied with opposite currents andpreferably located behind the cathode 14 so as to reduce or even cancelthe axial magnetic field on the surface of the cathode 14 and, hence,the canonical angular momentum spread. This technique permits the activecontrol of the beam velocity spread and also avoids potential arcingproblems discussed in the “Background” section. Further, this techniqueprovides the gyrotron gun 10 with the ability to operate in the largeand small-orbit modes of operation which may be further described withreference to FIGS. 6 and 7 illustrating results that were obtained fromthe aforementioned particle simulation study, already described withreference to FIGS. 3, 4 and 5.

FIG. 6 is composed of FIGS. 6(A) and 6(B) both of which show theelectron beam 42 normalized in axial momentum vs axial distance for twoseparate large-orbit simulations, similar to each other except for theamount of canonical angular momentum P_(θ) spread. FIG. 6(A) illustratesthe normalized axial momentum γβ_(z) shown by plot 90, wherein thebucking coils 38 and 40 are completely activated (no P_(θ) spread).Conversely, FIG. 6(B) illustrates the normalized axial momentum γβ_(z)shown by plot 92 resulting from the bucking coils 38 and 40 beingturned-off, thereby, providing for an axial velocity spread of 11.2% atα=1.83. The large velocity spread indicates that the P_(θ) quantity isone of the main contributors that cause for velocity spread inlarge-orbit beams.

FIG. 7 is composed of FIGS. 7(A), (B) and (C) all related to small-orbitoperations of gyrotron device 10. FIG. 7(A), (B) and (C) respectivelyillustrates plots 94, 96 and 98, wherein, respectively, the buckingcoils 38 and 40 are fully turned on, the bucking coils 38 and 40 arepartially turned on, and the bucking coils 38 and 40 are turned off. Theplot 94 indicates a final velocity spread of 3.9% at α=1.4, the plot 96indicates a final velocity spread of 1.6% at α=1.35, and the plot 98indicates a final velocity spread of 14.5% at α=1.2. A comparisonbetween plots 94, 96 and 98 reveals that the gyrotron device 10, inparticular, the bucking coils 38 and 40 act as a means for controllingthe velocity spread related to the small-orbit beam operation, and alsothat this velocity spread may be advantageously adjusted for variousapplications by the practice of this invention.

It should now be appreciated that the practice of the present inventionprovides for a means for controlling the axial velocity v_(z) spread,the gyrating electron transverse-to-axial velocity ratio α, as well asthe electron beam guiding center, r_(g). These factors are controlled bythe diode region 56, the double-cusp region 58, and the adiabaticcompression region 60 carrying an adjustable and predetermined magneticprofile 64.

It should therefore readily be understood that many modifications andvariations of the present invention are possible within the purview ofthe claimed invention. It is, therefore, to be understood that, withinthe scope of the appended claims, the invention may be practicedotherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A system for generating and forming a beam ofelectrons comprising: (a) a vacuum envelope forming a tunnel havingentrance, intermediate and exit sections with the intermediate havingfirst and second portions; (b) first, second and third field coils withthe first being situated at said entrance section and the second andthird being situated at said intermediate section; (c) first and secondmeans for establishing an abrupt change in a magnetic field, said firstand second abrupt change means being respectively interposed betweensaid first and second field coils and said second and third field coils,said first field coil and said first abrupt change means comprising afirst operating region, said second field coil and said first and secondabrupt change means comprising a second operating region and said thirdfield coil and said second abrupt change means comprising a thirdoperating region; (d) a cathode located in said entrance section andcapable of emitting a beam of electrons when subjected to the presenceof a relatively high voltage pulse; and (e) an anode located in saidentrance section and spaced apart from said cathode and capable ofattracting said beam of electrons.
 2. The system for generating andforming an electron beam according to claim 1 further comprising a pairof bucking coils located near said cathode.
 3. The system for generatingand forming an electron beam according to claim 1 further comprising aninteraction circuit connected to said exit portion and extractingmicrowave energy from said beam of electrons.
 4. The system forgenerating and forming an electron beam according to claim 1, whereinsaid cathode is thermionic or comprises other electron emitting devices.5. The system for generating and forming an electron beam according toclaim 1, wherein said cathode and said anode both have an annular shape.6. The system for generating and forming an electron beam according toclaim 1, wherein said cathode and anode are offset from each other andhave protrusions interposed therebetween so that said beam ofattractable electrons are diverted away from said anode and directedtoward said intermediate section.
 7. The system for generating andforming an electron beam according to claim 1, wherein said vacuumenvelope, said first, second and third field coils and said first andsecond means for establishing an abrupt change in a magnetic fieldcomprise a gyrotron device having a predetermined operating period witha corresponding predetermined wavelength and said first and second meansfor establishing an abrupt change are spaced apart from each other by adistance corresponding to one-half of said predetermined wavelength. 8.A method using a gyrotron gun having a cathode that emits a beam ofelectrons when subjected to the presence of a relatively high voltagepulse and an anode spaced apart from said cathode and energized withrespect to said cathode so that said beam of electrons are attractabletoward said anode, said gyrotron gun generating and forming a beam ofelectrons manifesting electron gyrating around a guiding center andhaving rotational energy, said beam-forming being determined by at leasttwo factors: (1) the transverse-to-axial velocity ratio, α, of thegyrating electron beam; and (2) the position of the electron's meanguiding center, r_(g), said method comprising the steps of: (a)providing a vacuum envelope that forms a tunnel having entrance,intermediate and exit sections with the intermediate section havingfirst and second portions; (b) arranging first, second and third fieldcoils with the first being situated at said entrance section and thesecond and third being situated at said intermediate section; (c)interposing first and second means for establishing an abrupt change ina magnetic field, said imposition placing said first means forestablishing an electric field between said first and second field coilsand said second means for establishing an abrupt change in a magneticfield between said second and third field coils, said first field coiland said first abrupt changing means comprising a first operatingregion, said second field coil and said first and second abrupt changemeans comprising a second operating region, and said third field coiland said second abrupt change means comprising a third operating region;(d) adjusting the first field coil to supply the strength of an axialmagnetic field corresponding to a first predetermined quantity f₁ insaid first operating region; (e) adjusting the second field coil tosupply the strength of an axial magnetic field corresponding to a secondpredetermined quantity f₂ in said second operating region; (f) adjustingthe third field coil to supply the strength of an axial magnetic fieldcorresponding to a third predetermined quantity f₃ in said thirdoperating region; (g) adjusting said f₁ and f₂ quantities to one of thefollowing relationships: (1) (f₁=f₂); (2) (|f₁|=−|f₂|) and (3) (f₁≠f₂).9. The method of using a gyrotron gun according to claim 8, wherein saidbeam-forming is further determined by a third factor: (3) the spread ofthe axial velocity v_(z) of said beam and wherein said method furthercomprises the steps of: (h) arranging a pair of bucking coils near saidcathode; and (i) adjusting the level of current in said bucking coils tocontrol said spread of axial velocity of said beam.